Limited proteolysis of rat brain tubulin yields two types of products, those derived from carboxy terminal cleavage of both subunits and those derived from a single internal cut of each subunit. These changes have major effects on polymerization of tubulin: carboxy terminal cleavage promotes polymerization whereas internal cleavage hinders assembly. The carboxy terminal-cleaved tubulin promotes the assembly of uncleaved tubulin and tissues may use this mechanism to facilitate polymerization under unfavorable conditions.